Measuring Chokes

Dear Technoid:

Help. With the rotten weather not much else to but raise shooting questions;

1. Am I right that chokes are based on the comparison to the barrel id e.g.; bore of .740, skeet would measure .735, bore .730 skeet would measure .725?

2. chokes could also be done by patterning; distance diameter and percentage?

3. Now the big one, its becoming increasingly popular around our area to see a lot of Cutts installed on the autoloaders. In making some measurements, the inside of the vented part measures 1.070 and the screw-in skeet choke measures .785. Do you know if this was derived by patterning and what’s your opinion of the Cutts?

I really enjoy your web site keep up the good work or at least the work

Thanks Larry

Dear Larry,

Waddayamean “at least the work”! This stuff is priceless. Pure gold. As close to the truth as you are going to get this side of a deodorant ad. It’s free too.

The amount of constriction of the choke is the DIFFERENCE between the bore diameter and the choke. A bore diameter of .725″ and a choke diameter of .720″ would give a constriction of .005″ or what is generally considered to be a “skeet” constriction in 12 gauge. Be aware that there is no universally accepted opinion on how much constriction equals a certain amount of choke.

Choke is actually a measure of PERFORMANCE, not just constriction. Example: A Modified choke in the US is generally considered to be one that throws 60% of its pattern into a 30″ circle at 40 yards (all 12 gauge chokes are patterned into 30″ at 40 yards). Very often .020″ of constriction is required to do this. Thus .020″ constriction is often equated with Modified choke. IC’s 50% is often obtained with .010″ and Full’s 70-75% with .035″.

That said, if you found a shell that patterned really tight and could get a 60% pattern out of it using only .010″ constriction (normally associated with Improved Cylinder), then that .010″ choke would be Modified when shot with that shell. Performance is what counts, not any kind of barrel measurement. Confused yet? Me too.

Obviously, manufacturers have to stamp some kind of choke marking on the barrel. Instead of being honest about it and stamping the numeric constriction on the choke, they take the easy way out and stamp a choke name on it. This is probably a good idea because the average shooters doesn’t really know what the constriction numbers mean, but he does have a vague idea that IC is near, Mod if middlin’ and Full is far. It is also easier for the manufacturer, ganging of tolerances being what they are. Bottom line: If you want to know what you have, you have to pattern with the shell that you are going to use.

Best regards,
Bruce Buck
Shotgun Reports Technoid

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2 Responses to Measuring Chokes

  1. Ted Cherry says:

    Dear Technoid… Re: Shotgun bore roundness

    Is there an industry standard as to the guarantee of the true roundness of shotgun bores? I know this is ‘splitting hairs’ when measuring bores and chokes to determine actual restrictions, but what roundness tolerances are there, if any, during barrel manufacturing? The majority of dial bore gages are of the three point contact type, which does not allow the detection of an out of round bore. A telescoping gage combined with a common micrometer would allow you to check the bore for roundness, but telescoping gages are clumsy at times when trying to get the ‘right feel’.

    Any comments on this…? Thanks, Ted C.


    • Bruce Buck says:

      Dear Ted,

      The true roundness of the shotgun bore? This may be one too many angels on the head of my already overloaded pin. For most of it, shotgun manufacture is a “close enough” business. Example: I was testing a gun from one of our major makers for barrel convergence, ie whether or not the two barrels shot to the same point of impact at a certain distance. The barrels on my test gun shot 8″ apart at 40 yards. I felt that this was awful and called the gunmaker. They told me that this was within their production standards. Aarrgghh!

      So, do you think that they are going to care much about barrel I.D.s that are a tiny bit out of round? OK, maybe that’s unfair. I’m sure that they do care. Some of the makers hammer forge their barrels, most others gun drill them. When you are using a drill, it’s sort of hard not to make a circular hole unless there is something wrong with the metal. Hammer forging over a mandrel might allow a touch of out-of-roundness, but I really don’t see how it would affect patterns unless is was a grossly gross situation.

      While I don’t know much about checking the I.D.s of barrels for roundness, the better makers sure do spend a little bit of time getting the barrels straight. You’be be amazed (or perhaps not) as to how crooked a thin barrel can get when subjected to the heat of forming the tube. The barrels are straightened by eye and hand, not by machine. Apparently the human eye is superior when it comes to lining up the visual rings inside the barrel to assure straightness.

      Best regards,
      Bruce Buck
      Shotgun Report’s Technoid


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